magnitude or distance, doth it in the same way that words suggest the
notions to which they are annexed. Now, it is known a word pronounced
with certain circumstances, or in a certain context with other words,
hath not always the same import and signification that it hath when
pronounced in some other circumstances or different context of words. The
very same visible appearance as to faintness and all other respects, if
placed on high, shall not suggest the same magnitude that it would if it
were seen at an equal distance on a level with the eye. The reason
whereof is that we are rarely accustomed to view objects at a great
height; our concerns lie among things situated rather before than above
us, and accordingly our eyes are not placed on the top of our heads, but
in such a position as is most convenient for us to see distant objects
standing in our way. And this situation of them being a circumstance
which usually attends the vision of distant objects, we may from hence
account for (what is commonly observed) an object's appearing of
different magnitude, even with respect to its horizontal extension, on
the top of a steeple, for example, an hundred feet high to one standing
below, from what it would if placed at an hundred feet distance on a
level with his eye. For it hath been shown that the judgment we make on
the magnitude of a thing depends not on the visible appearance alone, but
also on divers other circumstances, any one of which being omitted or
varied may suffice to make some alteration in our judgment. Hence, the
circumstances of viewing a distant object in such a situation as is
usual, and suits with the ordinary posture of the head and eyes being
omitted, and instead thereof a different situation of the object, which
requires a different posture of the head taking place, it is not to be
wondered at if the magnitude be judged different: but it will be demanded
why an high object should constantly appear less than an equidistant low
object of the same dimensions, for so it is observed to be: it may indeed
be granted that the variation of some circumstances may vary the judgment
made on the magnitude of high objects, which we are less used to look at:
but it does not hence appear why they should be judged less rather than
greater? I answer that in case the magnitude of distant objects was
suggested by the extent of their visible appearance alone, and thought
proportional thereto, it is certain they would then be judged much less
than now they seem to be (VIDE sect. 79). But several circumstances
concurring to form the judgment we make on the magnitude of distant
objects, by means of which they appear far larger than others, whose
visible appearance hath an equal or even greater extension; it follows
that upon the change or omission of any of those circumstances which are
wont to attend the vision of distant objects, and so come to influence
the judgments made on their magnitude, they shall proportionably appear
less than otherwise they would. For any of those things that caused an
object to be thought greater than in proportion to its visible extension
being either omitted or applied without the usual circumstances, the
judgment depends more entirely on the visible extension, and consequently
the object must be judged less. Thus in the present case the situation of
the thing seen being different from what it usually is in those objects
we have occasion to view, and whose magnitude we observe, it follows that
the very same object, being an hundred feet high, should seem less than
if it was an hundred feet off on (or nearly on) a level with the eye.
What has been here set forth seems to me to have no small share in
contributing to magnify the appearance of the horizontal moon, and
deserves not to be passed over in the explication of it.
74. If we attentively consider the phenomenon before us, we shall find
the not discerning between the mediate and immediate objects of sight to
be the chief cause of the difficulty that occurs in the explication of
it. The magnitude of the visible moon, or that which is the proper and
immediate object of vision, is not greater when the moon is in the
horizon than when it is in the meridian. How comes it, therefore, to seem
greater in one situation than the other? What is it can put this cheat on
the understanding? It has no other perception of the moon than what it
gets by sight: and that which is seen is of the same extent, I say, the
visible appearance hath the same, or rather a less, magnitude when the
moon is viewed in the horizontal than when in the meridional position:
and yet it is esteemed greater in the former than in the latter. Herein
consists the difficulty, which doth vanish and admit of a most easy
solution, if we consider that as the visible moon is not greater in the
horizon than in the meridian, so neither is it thought to be so. It hath
been already shown that in any act of vision the visible object
absolutely, or in itself, is little taken notice of, the mind still
carrying its view from that to some tangible ideas which have been
observed to be connected with it, and by that means come to be suggested
by it. So that when a thing is said to appear great or small, or whatever
estimate be made of the magnitude of any thing, this is meant not of the
visible but of the tangible object. This duly considered, it will be no
hard matter to reconcile the seeming contradiction there is, that the
moon should appear of a different bigness, the visible magnitude thereof
remaining still the same. For by sect. 56 the very same visible
extension, with a different faintness, shall suggest a different tangible
extension. When therefore the horizontal moon is said to appear greater
than the meridional moon, this must be understood not of a greater
visible extension, but a of greater tangible or real extension, which by
reason of the more than ordinary faintness of the visible appearance, is
suggested to the mind along with it.
75. Many attempts have been made by learned men to account for this
appearance. Gassendus, Descartes, Hobbes, and several others have
employed their thoughts on that subject; but how fruitless and
unsatisfactory their endeavours have been is sufficiently shown in THE
TRANSACTIONS,[Phil. Trans. Num. 187. p. 314] where you may
see their several opinions at large set forth and confuted, not without
some surprize at the gross blunders that ingenious men have been forced
into by endeavouring to reconcile this appearance with the ordinary
Principles of optics. Since the writing of which there hath been published
in the TRANSACTIONS [Numb. 187. P. 323] another paper relating to the same
affair by the celebrated Dr. Wallis, wherein he attempts to account for
that phenomenon which, though it seems not to contain anything new or
different from what had been said before by others, I shall nevertheless
consider in this place.
76. His opinion, in short, is this; we judge not of the magnitude of an
object by the visual angle alone, but by the visual angle in conjunction
with the distance. Hence, though the angle remain the same, or even
become less, yet if withal the distance seem to have been increased, the
object shall appear greater. Now, one way whereby we estimate the
distance of anything is by the number and extent of the intermediate
objects: when therefore the moon is seen in the horizon, the variety of
fields, houses, etc., together with the large prospect of the wide
extended land or sea that lies between the eye and the utmost limb of the
horizon, suggest unto the mind the idea of greater distance, and
consequently magnify the appearance. And this, according to Dr. Wallis,
is the true account of the extraordinary largeness attributed by the mind
to the horizontal moon at a time when the angle subtended by its diameter
is not one jot greater than it used to be.
77. With reference to this opinion, not to repeat what hath been already
said concerning distance, I shall only observe, FIRST, that if the
prospect of interjacent objects be that which suggests the idea of
farther distance, and this idea of farther distance be the cause that
brings into the mind the idea of greater magnitude, it should hence
follow that if one looked at the horizontal moon from behind a wall, it
would appear no bigger than ordinary. For in that case the wall
interposing cuts off all that prospect of sea and land, etc. which might
otherwise increase the apparent distance, and thereby the apparent
magnitude of the moon. Nor will it suffice to say the memory doth even
then suggest all that extent of land, etc., which lies within the
horizon; which suggestion occasions a sudden judgment of sense that the
moon is farther off and larger than usual. For ask any man who, from such
a station beholding the horizontal moon, shall think her greater than
usual, whether he hath at that time in his mind any idea of the
intermediate objects, or long tract of land that lies between his eye and
the extreme edge of the horizon? And whether it be that idea which is the
cause of his making the aforementioned judgment? He will, I suppose,
reply in the negative, and declare the horizontal moon shall appear
greater than the meridional, though he never thinks of all or any of
those things that lie between him and it. SECONDLY, it seems impossible
by this hypothesis to account for the moon's appearing in the very same
situation at one time greater than at another; which nevertheless has
been shown to be very agreeable to the principles we have laid down, and
receives a most easy and natural explication from them. For the further
clearing' up of this point it is to be observed that what we immediately
and properly see are only lights and colours in sundry situations and
shades and degrees of faintness and clearness, confusion and
distinctness. All which visible objects are only in the mind, nor do they
suggest ought external, whether distance or magnitude, otherwise than by
habitual connexion as words do things. We are also to remark that, beside
the straining of the eyes, and beside the vivid and faint, the distinct
and confused appearances (which, bearing some proportion to lines and
angles, have been substituted instead of them in the foregoing part of
this treatise), there are other means which suggest both distance and
magnitude; particularly the situation of visible points of objects, as
upper or lower; the one suggesting a farther distance and greater
magnitude, the other a nearer distance and lesser magnitude: all which is
an effect only of custom and experience; there being really nothing
intermediate in the line of distance between the uppermost and lowermost,
which are both equidistant, or rather at no distance from the eye, as
there is also nothing in upper or lower, which by necessary connexion
should suggest greater or lesser magnitude. Now, as these customary,
experimental means of suggesting distance do likewise suggest magnitude,
so they suggest the one as immediately as the other. I say they do not
(VIDE sect. 53) first suggest distance, and then leave the mind from
thence to infer or compute magnitude, jut suggest magnitude as
immediately and directly as they suggest distance.
78. This phenomenon of the horizontal moon is a clear instance of the
insufficiency of lines and angles for explaining the way wherein the mind
perceives and estimates the magnitude of outward objects. There is
nevertheless a use of computation by them in order to determine the
apparent magnitude of things, so far as they have a connexion with, and
are proportional to, those other ideas or perceptions which are the true
and immediate occasions that suggest to the mind the apparent magnitude
of things. But this in general may, I think, be observed concerning
mathematical computation in optics: that it can never be very precise and
exact since the judgments we make of the magnitude of external things do
often depend on several circumstances, which are not proportionable to,
or capable of being defined by, lines and angles.
79. From what has been said we may safely deduce this consequence; to
wit, that a man born blind and made to see would, at first opening of his
eyes, make a very different judgment of the magnitude of objects
intromitted by them from what others do. He would not consider the ideas
of sight with reference to, or as having any connexion with, the ideas of
touch: his view of them being entirely terminated within themselves, he
can no otherwise judge them great or small than as they contain a greater
or lesser number of visible points. Now, it being certain that any
visible point can cover or exclude from view only one other visible
point, it follows that whatever object intercepts the view of another
hath an equal number of visible points with it; and consequently they
shall both be thought by him to have the same magnitude. Hence it is
evident one in those circumstances would judge his thumb, with which he
might hide a tower or hinder its being seen, equal to that tower, or his
hand, the interposition whereof might conceal experimental means the
firmament from his view, equal to the firmament: how great an inequality
soever there may in our apprehensions seem to be betwixt those two
things, because of the customary and close connexion that has grown up in
our minds between the objects of sight and touch; whereby the very
different and distinct ideas of those two senses are so blended and
confounded together as to be mistaken for one and the same thing; out of
which prejudice we cannot easily extricate ourselves.
80. For the better explaining the nature of vision, and setting the
manner wherein we perceive magnitudes in a due light, I shall proceed to
make some observations concerning matters relating thereto, whereof the
want of reflexion, and duly separating between tangible and visible
ideas, is apt to create in us mistaken and confused notions. And FIRST, I
shall observe that the MINIMUM VISIBILE is exactly equal in all beings
whatsoever that are endowed with the visive faculty. No exquisite
formation of the eye, no peculiar sharpness of sight, can make it less in
one creature than in another; for it not being distinguishable into
parts, nor in any wise a consisting of them, it must necessarily be the
same to all. For suppose it otherwise, and that the MINIMUM VISIBILE of a
mite, for instance, be less than the MINIMUM VISIBILE of a man: the
latter therefore may by detraction of some part be made equal to the
former: it doth therefore consist of parts, which is inconsistent with
the notion of a MINIMUM VISIBILE or point.
81. It will perhaps be objected that the MINIMUM VISIBILE of a man doth
really and in itself contain parts whereby it surpasses that of a mite,
though they are not perceivable by the man. To which I answer, the
MINIMUM VISIBILE having (in like manner as all other the proper and
immediate objects of sight) been shown not to have any existence without
the mind of him who sees it, it follows there cannot be any pan of it
that is not actually perceived, and therefore visible. Now for any object
to contain distinct visible parts, and at the same time to be a MINIMUM
VISIBILE, is a manifest contradiction.
82. Of these visible points we see at all times an equal number. It is
every whit as great when our view is contracted and bounded by near
objects as when it is extended to larger and remoter. For it being
impossible that one MINIMUM VISIBILE should obscure or keep out of sight
mote than one other, it is a plain consequence that when my view is on
all sides bounded by the walls of my study see just as many visible
points as I could, in case that by the removal of the study-walls and all
other obstructions, I had a full prospect of the circumjacent fields,
mountains, sea, and open firmament: for so long as I am shut up within
the walls, by their interposition every point of the external objects is
covered from my view: but each point that is seen being able to cover or
exclude from sight one only other corresponding point, it follows that
whilst my sight is confined to those narrow walls I see as many points,
or MINIMA VISIBILIA, as I should were those walls away, by looking on all
the external objects whose prospect is intercepted by them. Whenever
therefore we are said to have a greater prospect at one time than
another, this must be understood with relation, not to the proper and
immediate, but the secondary and mediate objects of vision, which, as
hath been shown, properly belong to the touch.
83. The visive faculty considered with reference to its immediate objects
may be found to labour of two defects. FIRST, in respect of the extent or
number of visible points that are at once perceivable by it, which is
narrow and limited to a certain degree. It can take in at one view but a
certain determinate number of MINIMA VISIBILIA, beyond which it cannot
extend its prospect. Secondly, our sight is defective in that its view is
not only narrow, but also for the most part confused: of those things
that we take in at one prospect we can see but a few at once clearly and
unconfusedly: and the more we fix our sight on any one object, by so much
the darker and more indistinct shall the rest appear.
84. Corresponding to these two defects of sight, we may imagine as many
perfections, to wit, 1ST, that of comprehending in one view a greater
number of visible points. 2DLY, of being able to view them all equally
and at once with the utmost clearness and distinction. That those
perfections are not actually in some intelligences of a different order
and capacity from ours it is impossible for us to know.
85. In neither of those two ways do microscopes contribute to the
improvement of sight; for when we look through a microscope we neither
see more visible points, nor are the collateral points more distinct than
when we look with the naked eye at objects placed in a due distance. A
microscope brings us, as it were, into a new world: it presents us with a
new scene of visible objects quite different from what we behold with the
naked eye. But herein consists the most remarkable difference, to wit,
that whereas the objects perceived by the eye alone have a certain
connexion with tangible objects, whereby we are taught to foresee what
will ensue upon the approach or application of distant objects to the
parts of our own body, which much conduceth to its preservation, there is
not the like connexion between things tangible and those visible objects
that are perceived by help of a fine microscope.
86. Hence it is evident that were our eyes turned into the nature of
microscopes, we should not be much benefited by the change; we should be
deprived of the forementioned advantage we at present receive by the
visive faculty, and have left us only the empty amusement of seeing,
without any other benefit arising from it. But in that case, it will
perhaps be said, our sight would be endued with a far greater sharpness
and penetration than it now hath. But it is certain from what we have
already shown that the MINIMUM VISIBILE is never greater or lesser, but
in all cases constantly the same: and in the case of microscopical eyes I
see only this difference, to wit, that upon the ceasing of a certain
observable connexion betwixt the divers perceptions of sight and touch,
which before enabled us to regulate our actions by the eye, it would now
be rendered utterly unserviceable to that purpose.
87. Upon the whole it seems that if we consider the use and end of sight,
together with the present state and circumstances of our being, we shall
not find any great cause to complain of any defect or imperfection in it,
or easily conceive how it could be mended. With such admirable wisdom is
that faculty contrived, both for the pleasure and convenience of life.
88. Having finished what I intended to say concerning the distance and
magnitude of objects, I come now to treat of the manner wherein the mind
perceives by sight their situation. Among the discoveries of the last
age, it is reputed none of the least that the manner of vision hath been
more clearly explained than ever it had been before. There is at this day
no one ignorant that the pictures of external objects are painted on the
RETINA, or fund of the eye: that we can see nothing which is not so
painted: and that, according as the picture is more distinct or confused,
so also is the perception we have of the object: but then in this
explication of vision there occurs one mighty difficulty. The objects are
painted in an inverted order on the bottom of the eye: the upper part of
any object being painted on the lower part of the eye, and the lower part
of the object on the upper part of the eye: and so also as to right and
left. Since therefore the pictures are thus inverted, it is demanded how
it comes to pass that we see the objects erect and in their natural
posture?
89. In answer to this difficulty we are told that the mind, perceiving an
impulse of a ray of light on the upper part of the eye, considers this
ray as coming in a direct line from the lower part of the object; and in
like manner tracing the ray that strikes on the lower part of the eye, it
is directed to the upper part of the object. Thus in the adjacent figure,
C, the lower point of the object ABC, is projected on C the upper part of
the eye. So likewise the highest point A is projected on A the lowest
part of the eye, which makes the representation CBA inverted: but the
mind considering the stroke that is made on C as coming in the straight
line CC from the lower end of the object; and the stroke or impulse on a
as coming in the line AA from the upper end of the object, is directed to
make a right judgment of the situation of the object ABC, notwithstanding
the picture of it is inverted. This is illustrated by conceiving a blind
man who, holding in his hands two sticks that cross each other, doth with
them touch the extremities of an object, placed in a perpendicular
situation. It is certain this man will judge that to be the upper part of
the object which he touches with the stick held in the undermost hand,
and that to be the lower part of the object which he touches with the
stick in his uppermost hand. This is the common explication of the erect
appearance of objects, which is generally received and acquiesced in,
being (as Mr. Molyneux tells us [Diopt. par. 2. c. 7. P. 289.]) 'allowed
by all men as satisfactory'.
90. But this account to me does not seem in any degree true. Did I
perceive those impulses, decussations, and directions of the rays of
light in like manner as hath been set forth, then indeed it would not be
altogether void of probability. And there might be some pretence for the
comparison of the blind man and his cross sticks. But the case is far
otherwise. I know very well that I perceive no such thing. And of
consequence I cannot thereby make an estimate of the situation of
objects. I appeal to anyone's experience, whether he be conscious to
himself that he thinks on the intersection made by the radious [SIC]
pencils, or pursues the impulses they give in right lines, whenever he
perceives by sight the position of any object? To me it seems evident
that crossing and tracing of the rays is never thought on by children,
idiots, or in truth by any other, save only those who have applied
themselves to the study of optics. And for the mind to judge of the
situation of objects by those things without perceiving them, or to
perceive them without knowing it, is equally beyond my comprehension. Add
to this that the explaining the manner of vision by the example of cross
sticks and hunting for the object along the axes of the radious pencils,
doth suppose the proper objects of sight to be perceived at a distance
from us, contrary to what hath been demonstrated.
91. It remains, therefore, that we look for some other explication of
this difficulty: and I believe it not impossible to find one, provided we
examine it to the bottom, and carefully distinguish between the ideas of
sight and touch; which cannot be too oft inculcated in treating of
vision: but more especially throughout the consideration of this affair
we ought to carry that distinction in our thoughts: for that from want of
a right understanding thereof the difficulty of explaining erect vision
seems chiefly to arise.
92. In order to disentangle our minds from whatever prejudices we may
entertain with relation to the subject in hand, nothing seems more
apposite than the taking into our thoughts the case of one born blind,
and afterwards, when grown up, made to see. And though, perhaps, it may
not be an easy task to divest ourselves entirely of the experience
received from sight, so as to be able to put our thoughts exactly in the
posture of such a one's, we must, nevertheless, as far as possible,
endeavour to frame true conceptions of what might reasonably be supposed
to pass in his mind.
93. It is certain that a man actually blind, and who had continued so
from his birth, would by the sense of feeling attain to have ideas of
upper and lower. By the motion of his hand he might discern the situation
of any tangible object placed within his FI reach. That part on which he
felt himself supported, or towards which he perceived his body to
gravitate, he would term lower, and the contrary to this upper; and
accordingly denominate whatsoever objects he touched.
94. But then, whatever judgments he makes concerning the situation of
objects are confined to those only that are perceivable by touch. All
those things that are intangible and of a spiritual nature, his thoughts
and desires, his passions, and in general all the modifications of the
soul, to these he would never apply the terms UPPER and LOWER, except
only in a metaphorical sense. He may, perhaps, by way of allusion, speak
of high or low thoughts: but those terms in their proper signification
would never be applied to anything that was not conceived to exist
without the mind. For a man born blind, and remaining in the same state,
could mean nothing else by the words HIGHER and LOWER than a greater or
lesser distance from the earth; which distance he would measure by the
motion or application of his hand or some other part of his body. It is
therefore evident that all those things which, in respect of each other,
would by him be thought higher or lower, must be such as were conceived
to exist without his mind, in the ambient space.
95. Whence it plainly follows that such a one, if we suppose him made to
see, would not at first sight think anything he saw was high or low,
erect or inverted; for it hath been already demonstrated in sect. 41 that
he would not think the things he perceived by sight to be at any distance
from him, or without his mind. The objects to which he had hitherto been
used to apply the terms UP and DOWN, HIGH and LOW, were such only as
affected or were some way perceived by his couch: but the proper objects
of vision make a new set of ideas, perfectly distinct and different from
the former, and which can in no sort make themselves perceived by touch.
There is, therefore, nothing at all that could induce him to think those
terms applicable to them: nor would he ever think it till such time as he
had observed their connexion with tangible objects, and the same
prejudice began to insinuate itself into his understanding, which from
their infancy had grown up in the understandings of other men.